Plywood thickness under tile

After reading a bunch of posts on this subject, 1 1/4" thickness seems to be the minimum thickness recommended, (usually 3/4" plus 1/2"). There is already 5/8" plywood underlayment, can I add another 5/8" sheet and have enough strength for tile?

I also had about 1/4" to 1/3" out of flat problem, detected with a 4' metal straight edge, on the sub-flooring. Most of the problem occurs in a shorter distance. (1 or 2 feet). I removed a section of the sub-floor and found one of the floor joists was at least 1/4" high. I shaved it down to match the surrounding area and replaced the plywood..........My question is..How much flatness variation is OK, and over how short of a span. Will be using large format tile. (12" x 12", or 8" x 24"). Thanks
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It's nice to get as stiff a subfloor as possible, but in fact, the MINIMUM TCNA guidelines for a tiled floor over 16" OC joists is 5/8" with T&G (or blocked seams). More is nice, and is required if the tile is natural stone (second layer a minimum of 3/8"). Many pros would not even consider laying tile over 5/8" (except a low-cost new house, or a flip, maybe), but it will hold up if that layer is properly installed and in good condition. It's always nice to get a bit over the minimum, and I'd not have any problems installing over a good layer of 3/4".

I'd have to look it up, but for flatness, I think the standard is 1/8" in 10'.

But, subflooring is only one part of the equation...the joists must also meet the minimum (preferably better) deflection standard of L/360 for ceramic (porcelain is a type of ceramic and has the same requirements). If that's deficient, no amount of subflooring will provide a reliable result.

Thanks for the info. I think I'll be OK with the deflection. I have 9" joists, 14" O.C., with a span of about 10'. I'll be using Laticrete thinsets, with Ditra also.
As far as the floor flatness issue, can any of it be compensated for with thinset thickness at tile install?

FWIW, the Robinson floor test that is the basis for the approved tile installation methods makes no distinction between the tile size or shape, only in composition (ceramic or natural stone). So, the industry standards say, properly installed, L/360 works for ceramic, that's okay by me...they have the test results and industry concurrence. Certainly, more is nice.

Cross-bracing does help with POINT loads, but has NOTHING to do with a distributed load...the joists are only so strong, and if a load is applied to all of them, cross-bracing adds little. What cross-bracing does do, as I said, is help with point loads AND it helps the existing joists MAINTAIN their load-carrying capacity, since it keeps them parallel with the load, and thus upright and able to use their full strength rather than potentially twisting and presenting a less than ideal load path. It can also change the resonant frequency of the floor, which is more of a perceived strength than a real one. IOW, a well-braced floor will feel stronger than any actual change, which is not a bad thing, but none of the industry standards require it.

FWIW, when I was at the CTEF, where they do the Robinson floor test to verify the installation manual for TCNA and the manufacturers, they had some floor tile that was something like 2'x4'...same results, size doesn't matter. Installation is crucial, you must get good coverage of thinset and have a floor within the specified strength. The bigger the tile, the more important it is to burn thinset onto the back and ensure you have nice even coverage AND set it well to get proper coverage, which is the key thing for any tile, but crucial on a larger tile where a void could lead to cracking.

Thinset, by design, is not intended to be applied very thick. Depending on how big the deviations from flat your floor is, if it does not exceed the max depth for the thinset you are using, yes, you can use it to level as you go. But, it is MUCH harder to do this while maintaining full coverage or risking the tile sinking into the thinset if you happen to lean on it (or just gravity of it's own weight) if you aren't careful. In this instance, you might want to consider something like the Tuscan Leveling system or the QEP clips and wedges (there are others, too) that actually hold the adjacent tiles perfectly level with each other. While the QEP stuff is a little harder to use, if you aren't doing it often, you don't need to buy the tool required with the Tuscan system, and you don't need to think ahead as far (their clips MUST be soaked in water overnight before use).

If you want to fix it with thin-set you will need a medium bed mortar. We often set this way.

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Better wait for Jim to double check all this with his notes - I think he is going to tell us non-modfied thin-set is stronger than modified next.
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Hi John, just peeked in here again; Been real busy, but I missed this place. I see you still have a way with words, so don't hold anything back

If John would read what I said, I also suggested the QEP LASH system so you DIDN'T have to buy the tool, but both will work. For a pro, using it often, I think the consistency you get with the tool is faster and more reliable, but the LASH system works fine if you use it properly.

I've never said a dryset was stronger than a modified, BUT, there's always the question of how much do you need, and what is the application. There's a very good technical reason why Schluter calls for an unmodified between the tile and the mat...independent tests have shown that a modified was still not up to the specs (i.e., still wet) after two months when they stopped looking. How long are you willing to wait for that extra strength that is not needed? The bond between the Ditra and the floor will break FAR before the bond between the tile and the mortar. Since neither the tile nor the mortar holding it into the Ditra mat is physically connected to the floor, you get PLENTY of allowance for expansion and contraction, so the argument that you must have a modified for that tile is kind of moot IF you understand how it works!

All floors are designed (or should be!) for a dead load and a live load. Exceeding the sum of the dead and live load means the floor is overloaded. Anything attached to the joists or hung from them is a dead load. Adding extra layers of ply, blocking, etc., come at a cost - raising the dead load on the structure. A tiled installation can be quite heavy when you start to add layers that are not required. Exceeding the load design on a floor doesn't mean immediate failure...I've read some forensic test results on failed tile installations that occurred after something like 10-years that was finally attributed to excessive dead loads. A common design for residences is a 40# dead load and a 10# live load per square foot. In a bathroom, your total floor load can add up very fast. Keep in mind, a non-loadbearing wall is also a dead load applied to the floor as is stuff like heating ducts, drain lines, whatever subflooring you have, and the ceiling drywall of the floor below since all of that is being supported by the joists. Then, throw in a fully tiled shower with a full-conventional mudded pan, and maybe a 6' soaking tub and you can exceed the dead+live load of the floor. Throw in a nice granite counter on a big vanity (at around 250#/cuft) and it starts to add up really fast. This often doesn't show up for years, if it does at all. But, the object here is to know when enough is enough. Adding extra stuff than the industry has tested and approved that works COULD lead to a failure further down the road. The belt and suspenders approach may just come back to bite you...it is important to build strong, but it is also important to know when enough is enough. I don't think John realizes that. He really likes belt and suspender redundancy...

ALL cement products require moisture to properly cure and attain their full strength. Putting it between a waterproof membrane and a nearly impervious tile means any moisture that is there doesn't evaporate (well, at a typical 1% max absorption rate, some will slowly get through porcelain, but maybe none through glass tile) and can be incorporated into the cement's crystalline structure. So, holding what moisture IS there, means the concrete gets to fully achieve its strength AND (this is the big thing that at least Schluter realizes) you get a guaranteed, repeatable, reliable result in a realistic timeframe. Sort of like wetting or covering a newly poured slab. But, a modified also needs to DRY. A modified thinset does not attain it's full strength until it both cures AND dries. The study showed that it still had not dried its modifiers after two months, when they stopped. It's a different story whey you install it on something like say cbu...that stuff will wick away the moisture just fine, and in fact, if you don't prewet it, it will wick it away TOO fast, before the thinset can cure and create a weak bond. This is also one reason why you use a modified underneath Ditra - it CAN dry out. Nowhere in the section you quoted did I even mention a modified thinset, so to characterize that as unmodified is stronger has no basis at all. All I said was when using a membrane like Kerdi (or Noble's, or anyone's), the mechanical bond of the membrane to the wall or floor is MUCH less than a dryset mortar's bond to the tile. So, why is it so 'necessary' for strength? Installed over cbu, it may be nice partly because there's more mass and no cleavage layer between the wall and the tile.

I never said the room ended up on the lower floor...I'm talking about cracks in the floor that didn't happen for years after the install. A dead load in excess of the design means the floor doesn't rebound, and takes a set. The longer that load is applied, it will continue to bend until it reaches some equilibrium. This can increase as live loads are added and removed, thus the reason why it may take a long time to become noticeable. The joists are only so elastic. Ever notice an old building's roof where it has dips between each rafter? Likely overloaded (snow?) periodically. It didn't bend that far the first season, it bent a little more as that load was applied each time it was overloaded. Your floor will do the same thing if it is overloaded. It just takes time.

A bathroom can have a very significant load on the floor. Run the numbers by yourself. See how close you get to the typical design build of a residential floor of 40+10. When you throw in the partition walls, all of the subflooring, the tile underlayment, the mortar, the fasteners, the tile, the toilet, the shower, the vanity, the heavy granite countertop...it all adds up. Then, in your fancy homes you like to work on, throw in a 100# chandelier hanging from that floor below and a real plaster ceiling, and your dead load margin goes down even quicker.

Throwing in extra stuff just because it makes you feel better, is not always the right decision...the cost/benefit/risk needs to be taken into account.

For those of you newbies that want a bit of backround....
Mr Whipple:
Banned from at least 2 tile forums 1) John Bridge and 2)Contractor Talk
Denied from joining TileGeeks on Facebook
Told to cease and desist his slanderous rants from a major manufacturer of tile products(read his posts and you can guess what one)
Does very little tile himself. Stands around watching and takes credit for his setters work.
He is a plumber
Looked at as a complete and utter lunatic from everyone in the tile industry.
Thinks, in his own mind, he is a major player in the world of tile.

You gotta love someone who thinks he knows better than hundreds of engineers, multiple testing agencies, thousands and thousands of installs, and years and years of experience, that his way is the only way to do something, and better than anyone else's.

You gotta love someone who thinks he knows better than hundreds of engineers, multiple testing agencies, thousands and thousands of installs, and years and years of experience, that his way is the only way to do something, and better than anyone else's.

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And dont question it!
Keep giving good info Jim based on facts not "because i said so".

That report was read years ago, but the details stuck with me. Not worth trying to find. Can you find that old magazine or article you read 5-10 years ago?

A couple of personal examples of stressing the system:
1. For the first 8-years or so I had a waterbed in my bedroom. I ran the numbers, and the bed was about 30#/sqft. Then add in the frame, the carpet, the subflooring, and it was close to the 40# dead load. Not all that much in the rest of the room, but it was not empty. It so happens that each wall is load bearing around the perimeter of the room. There's a non-load-bearing partition wall slightly to the side of where the bed was placed under this room. The wall ran parallel with the joists, and was not on one. I noticed a crack in the opening between rooms down below in the drywall, and when I did some remodeling of the rooms below found that that non-load-bearing wall had been pressed about 3/4" down...it had literally bent the subflooring into a z-shape, since it was fairly close to a joist. This from a design/build to the specs of 40/10 with a load less than the design. It took a bunch of years to occur. The wood has some elasticity and plasticity to it, but apply a load at or near its design limits, and when it bends, it does not bend back. The plasticity of it allowed it to bend rather than break because the load was applied continually over a long time. The crack didn't close up when I replaced the mattress with a conventional one, either...the stuff had taken a set.

2. Have a small deck off the dining room that had cantilevered joists - about 3/4'rs under the interior, and 1/4 outside (sitting over the outside wall). Edges supported with diagonals to the walls. Over about 15-years, probably from a snow load that was intermittent, the dining room crowned about 3"! Call it an inverse load, if you will. The deck was redesigned by an engineer, and replaced. The load path strengthened and hung from the rim joist with steel shell for support. No further problems.

Both of these are to illustrate that it can take some time to make any observation of overloading a structure. You hear of reports of decks falling down, partly because they tend to have an accumulation of people partying, but overloaded is still overloaded, and things can only bend so far. My personal examples don't show catastrophic failures, but that things do bend, and eventually if the load is applied continuously and over the design strength, it won't spring back.

Let's take a shower, from FL's state design guidelines (the first ones I found on a quick search), 1" mudbed is about 12#/sqft. In a shower of any size, using a conventional clamping drain, it probably averages 3", so you have 36#/sqft. Now, throw on some tile, averaging say 4#/sqft, you're at the max design dead load of any room without the subflooring (or extra blocking you want). THen, add in that extra layer of plywood, the room, already overweight, now is even more overloaded. We haven't added in the partition walls for the side of the shower....let's say it's 4x8 and you've tiled to the ceiling...that's about 320# if you're using say granite or marble on only a few square feet and we haven't included the cbu, studs, waterproofing, etc., you need to actually build a shower wall. In your high-end house, let's say you have CI drain lines...throw in another few hundred pounds for the bathroom and that big, two sink vanity with 3cm granite counter, maybe another thousand pounds by the time the silly thing is loaded up and you count the two sinks, faucets, tiled or stone backsplash, etc. Add in the water closet (toilet), with the tank full, probably close to 100#. Maybe add a bidet. THen, cause it's an upscale house, you have a nice big CI whirlpool (no cheesy plastic tubs here!) at 400# or more then add a mortar bed, and a tiled raised platform. It all adds up. Average this out over the room, and you can easily exceed the 40+10, and you haven't filled the tub or put anyone into it.

So, as I said, there's a cost/benefit/risk to any build. Adding in more when you don't need it is not good design, it's dangerous with short sight. These things do not happen overnight...they take time, like that barn in the country with the wavy ridge line. They happen way past the typical 1-year warranty of most installs, and may take decades. Far enough into the future, where it gets shrugged off, or they've decided to remodel anyway...but, they are real.

Wood in Europe is usually reserved for decorative surfaces, and is not used as structural very often today. They don't have the luxury of large forests they're willing to cut down anywhere near the scale that is typical in the NA continent. They tend to have lots more brick and concrete structures. This tends to have MUCH higher design load capability than the typical stick built house in North America. It doesn't seem strange to me that they may have adopted different methods to do things...weight doesn't factor into things anywhere near as much. Trying to emulate their methods when the infrastructure is different can be problematic. In my view, anyway, that's one good reason why some of the installation instructions differ in region to region. That does NOT mean one is better than the other. You have to know when enough is enough and adapt to the situation. Telling the world your way is the only way and any other is junk is pretty narrow minded. Your knocking the manufacturer's instructions as unreliable and incorrect when testing has proven over and over again that it works is pretty pompous. Advocating belt and suspenders, when it adds cost, time, materials, weight just doesn't add up except in your mind.

Enough for now...this is really getting old. In my view, needed, though, as counter-point. Just read the manufacturer's instructions, follow them. If you don't understand, seek guidance. It works as designed.

John, prior to the post in this thread, I'd not heard of that website, as if it would make a difference, or was any of your business. You don't believe anything I say, regardless. Dismissing a comment, doesn't make it incorrect, either. The novel you refer to was to illustrate that high loads take time to distort the wooden structure, and once distorted, if you go beyond the elastic limits of the materials used, they do not spring back. The deflection of the joists for the typical L/360 or L/720 that get talked about are based on the total load design (usually 40+10). The deflection will progressively increase if your load exceeds the design spec. Eventually, something breaks, and if there's tile involved, it starts to show up as cracks. This can take years, way beyond the typical 1-year warranty, and likely beyond even a 10-year one. Exceeding requirements is a cost/benefit/risk balancing act...you have to take into account all variables. Extra stuff that ends up exceeding the design limits of the suspended floor count as a big minus, not the plus you imply.

As an aside, discussing overloaded structures, last fall I was in Salisbury Cathedral in England. Maybe 400-years ago, they added onto the bell tower and added some granite support columns (probably about 6' in diameter, not insignificant - think there may have been six of them, but don't remember exactly). Over those 400-years or so, those granite columns have a significant visible bow to them, as they were not sufficient for the added load. They certainly didn't have load tables or computers to help them figure things out back then, but it certainly LOOKED sufficient...your stuff may LOOK sufficient, but when you add up all the weight you're adding in the room with the stuff the industry says you do not need, it comes as a risk. Extra ply layers aids deflection between the joists, and is an insignificant benefit along the joists. If the structure already meets or exceeds the industry standards for deflection, why add it?

FWIW, 1.25" of drypack is about 15#/sqft. Well within the limits of design if you didn't go crazy with extra layers of whatever above what is required.

You do not seem to get the point, so I'll try to make it simpler: most wooden framed houses in the USA (and probably Canada) are built with a design load of 40#/sqft dead load and a 10# live load. If the architect is forward looking, or the design calls for especially heavy loads, he may strengthen the structure (like maybe in a bathroom, or the music room where there will be a full-sized grand piano, or a library where there will be lots of full bookshelves, or where you want to put a 1,000g salt water fish tank). Even things you wouldn't expect to bend (like the granite columns in the Cathedral I mentioned) can bend without breaking over time. Had those been the joists holding your pretty bathroom up, things would certainly start to show it. Adding extra stuff, when it is not necessary is NOT good design if it stresses and especially if it exceeds the design load of the structure.

There is a cost/risk/benefit relationship in all decisions...I do not believe in your evaluation of this...too short-sighted.